Mo-Bi-Fe catalyst series is known as an oxidation catalyst series. The performance of this kind of catalyst such as activity and selectivity is not very satisfactory. Chinese Patent 87103455.7 and U.S. Pat. No. 4,162,234 disclosed a catalyst, in which alkali metals such as Na, K, Rb, Cs, etc. were added to improve the activity and abrasion-resistance of the catalyst. Only one alkali metal element was incorporated into the above-mentioned catalysts. Japanese Patent No. 58-2232 (corresponding to U.S. Pat. No. 4,228,098) and Chinese Patent 86101301 disclosed a fluidized-bed catalyst for preparing acrylonitrile, in which a part of molybdenum component was replaced by tungsten component to improve the catalyst selectivity for acrylonitrile. The improvement in the activity and selectivity of the disclosed catalysts for preparing acrylonitrile is not significant and the processing capacity of the reactor is still limited.